Assuming you are using the marketplace module ‘Iframe’, and added the widget ‘Iframe component’ to a page, then in the widget’s properties go to tab Appearance, in attribute ‘Class’ add a classname, say ‘iframewidget’. Then go to ‘App’ → Styling → web → main.scss and add
.iframewidget{
border: 1em solid red;
}
You can use add a class or style in the Appearance
iframe {
border: 1em solid red;
}